Use integer bins for integer data in HistogramWidget

[p-j-smith]

Part of #239

• Originally part of Add widgets for setting histogram bins #242 but separated out for easier reviewing
• set the dtype for np.linspace when creating bins for HistogramWidget (so integer bin limits are used for integer images)
• this caused the test_histogram_2D test to fail (as the test layer has integer data but float bins were being used) so I've updated the baseline image